Programming and Problem Solving with Java

1.5 What Kinds of Instructions Can Be Written in a Programming Language? | 15

Source

code

Code listing,

possibly with

error messages

Machine language

version of

source code

(object code)

Input

data Results

Computer executes

machine language

version of

source code

Computer executes

COMPILATION compiler code

EXECUTION

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Figure 1.7 Compilation/Execution

ply to illustrate the difference between the traditional compile–execute steps and the com-
pile–interpret steps used with Java.
Viewed from a different perspective, the JVM makes the computer look like
a different computer, one that has Bytecode as its machine language. The com-
puter itself hasn’t changed—it remains the same collection of electronic cir-
cuits—but the JVM makes it appearto be a different machine. When a program is
used to make one computer act like another computer, we call it a virtual machine.
For convenience, we may refer to the computer as “executing a Java application,”
but keep in mind this is just shorthand for saying that “the computer is execut-
ing the JVM running a Java application.”

1.5 What Kinds of Instructions Can Be Written in a Programming

Language?

The instructions in a programming language reflect the operations a computer can perform:

 A computer can transfer data from one place to another.

 A computer can input data from an input device (a keyboard or mouse, for exam-

ple) and output data to an output device (a screen, for example).

Virtual machine A program

that makes one computer act

like another